Lineage commitment, differentiation, and self-renewal of hematopoietic stem cells are likely to involve specific changes in patterns of gene expression under the control of regulatory transcription factors. These altered patterns of gene expression are important, not only in producing cell-type specific proteins, but also in determining if, and when, cells continue to proliferative or to exit the cell cycle. The Myc transcription factor has been shown to play an important role in this decision in hematopoietic cells. Earlier work has demonstrated that Myc functions as part of a network of interacting proteins which include Myc's obligate dimerization partner, Max, as well as other Max interacting proteins, including the Mad family of transcriptional repressors. The Myc network is known to be involved in proliferation, differentiation, apoptosis and neoplasia. Work during the previous grant period has pointed to a role for Mad in hematopoietic cell differentiation and has led to a better understanding of the function of Myc. In this application we propose to extend these studies in order to elucidate the function of these proteins in hematopoietic cells. In Specific Aims 1 and 2 we plan to employ targeted deletions, as well transgenic in hematopoietic cells. In Specific Aims 1 and 2 we plan to employ targeted deletions, as well transgenic expression, or murine mad family genes to study hematopoiesis in vivo. In Aim 1 we propose to analyze the role of Myc and Mad transcriptional activities in hematopoietic development by examining their activities in the context of the known signaling pathways that control lymphopoiesis. The results of these studies will be used in Aim 2 to guide experiments on the role of Mad in the regulation of hematopoietic stem cell pool size, growth factor sensitivity, and lineage commitment. In Specific Aim 3 we will use oligonucleotide microarray analysis to identify genes differentially regulated by Myc and Mad in hematopoietic cells. Because Myc and Mad proteins appear to regulate transcription through alterations in histone acetylation, this work will help correlate hematopoietic differentiation with specific changes in gene expression and chromatin structure.